Amplifiers are widely used in analog circuits, particularly for analog-to-digital converters (“ADCs”). ADCs convert an analog signal to a digital signal by sampling the analog signal, processing it, and converting it to a digital signal. One important property of sampling circuits is bandwidth. Because an analog signal is one comprised of many frequencies, the larger the bandwidth of the sampling circuit, the greater the range of frequencies that may be captured by the sampling circuit. The analog signal is then passed to a signal processor that ultimately outputs a digital signal. In analog circuit design, there is a trade-off between bandwidth and noise. As bandwidth increases, more signal information can be processed. However, a larger bandwidth also permits more noise to pass through the filter, and more noise power must then be tolerated by the signal processor. Therefore, as a general design principle, an analog circuit should be of the minimum bandwidth necessary to process the required signal with specified accuracy.
Limiting the bandwidth of circuits to the minimum required for processing the input reduces the noise sampled, which is beneficial for signal processing. This is illustrated with two examples. First, the sampling bandwidth of a switched-capacitor sampling circuit, as in FIG. 1A, is typically designed to be three to four times the maximum required input frequency to obtain good gain and phase flatness over the input range. However, if the maximum required input frequency is reduced by a factor of four, then ideally the sampling bandwidth should also be reduced by a factor of four. The reduced bandwidth would minimize the total noise power sampled from any circuitry driving the sampling circuit. Second, when an amplifier in feedback is used as an active sampling circuit, as in FIG. 1B, the bandwidth is typically designed to be ten times the maximum required input frequency to maintain good AC linearity (i.e., a spurious-free output) for all required input frequencies. Although the required bandwidth is wider relative to the signal frequency of the switched-capacitor sampling circuit, the amplifier should still be designed with the minimum bandwidth necessary, also for noise considerations. Both examples illustrate that regardless of the application, bandwidth should be limited for noise considerations.
Information relevant to attempts to address these problems can be found in U.S. Pat. No. 7,298,151. However, in the circuit of U.S. Pat. No. 7,298,151 (shown as FIG. 2), the DC gain is limited by how large the load resistive element (RL) 208 can be made. This is because the DC gain from the input 204 of the transconductor to the output 205 is set by gmRL, where gm is the transconductance of the amplifier 212. Although there are many applications in which high DC gain is desirable, high DC gain is difficult to achieve, because DC gain is limited by how large RL can be made. If RL is too large, the circuit has issues related to parasitic capacitances including feedback loop instability.
In conventional circuit design, there are several ways to limit bandwidth. Two of the most common are described. In the first method, capacitance is increased to shunt some resistance in the signal path. In the example described above of the sampling circuit, as in FIG. 1A, this is accomplished by increasing the capacitance of sampling capacitor 114. In the example of the active sampling circuit, as in FIG. 1B, this is also accomplished by increasing the capacitance of sampling capacitor 106. However, one disadvantage of increasing capacitance, particularly at the input to the circuit, is the increased difficulty for the preceding circuit to drive the larger capacitive load, which may in turn increase the amount of overshoot and ringing at the output, and cause the driving circuit to be unstable.
Another way of limiting bandwidth in conventional circuit design is increasing resistance in series with the signal. In the circuit shown in FIG. 2, this is accomplished by increasing resistance in sampling switch 211. In an active sampling circuit, this is typically accomplished by decreasing the transconductance of the main amplifier input device. However, one disadvantage of increasing resistance is that it is accompanied by an increase in noise density. The noise of the sampling circuits in the presence of increased resistance is typically unchanged. Therefore, as the resistance increases, the bandwidth decreases, causing the noise density to increase. These effects offset each other. Furthermore, if some subsequent circuit has a fixed bandwidth, higher noise density results in increased overall noise.
Thus, there remains a need in the art for a way to limit the bandwidth of an amplifier that does not require either increasing the capacitance or increasing the resistance, which both negatively impact the performance of the circuit.